In digital multimedia transport, audio content is encoded in a variety of compression schemes. One of the most popular compression schemes is Dolby Digital (AC-3) audio. Dolby Digital AC-3 audio is outlined by ATSC standards document A/52B, Digital Audio Compression standard (AC-3, E-AC-3) Revision B, Jun. 14, 2005, which is hereby incorporated by reference. A feature supported by AC-3 audio is dialog normalization (“dialnorm”) value. Dialnorm value is an indication of audio program loudness. Currently the dialnorm value is calculated by special purpose equipment for an entire video program file. The dialnorm is encoded as a 5-bit field in every AC-3 audio frame. The valid values for dialnorm range from 1 to 31 that correspond to dialogue levels of −1 dB to −31 dB, respectively. The definition of the dialnorm as specified in the standard referenced above is an indication of the subjective loudness of normal spoken dialogue compared to 100 percent digital reproduction.
Dialnorm interpreted by the audio decoder as an indication of the audio content loudness. The dialnorm value can be interpreted by the audio decoder and is used to automatically adjust the volume of the decoded audio down to keep constant dialogue loudness. In general a decrease of 1 dB in the encoded dialnorm value (ranging from 1 to 31) would cause a drop of 1 dB in the decoder output gain level. A value of 31 in the dialnorm field indicates no gain loss due to dialogue normalization in the decoder output.